It has been hypothesized that the pathology of Alzheimer's Disease (AD) may involve oxidative stress resulting from the accumulation of free radicals, leading to excessive lipid peroxidation and neuronal degeneration in the brain. Antioxidants such as Vitamin E have been shown to protect cell membranes from oxidative damage by breaking the chain reaction of lipid peroxidation by donating an H+ to lipid peroxide radicals. In animal models, antioxidants reduce degeneration of hippocampal cells following cerebral ischemia, and enhance recovery of motor function in spinal cord injury. In cultural neurons, a-tocopherol reduced cell death associated with beta-amyloid protein. Previous research suggests that Idebenone may work as an antioxidant and may improve brain function. In this industry-sponsored trial, 500 patients with probable Alzheimer's Disease will be randomized to placebo, 120 240 or 360 mg of Idebenone given orally for 52 weeks. Primary outcome measures include changes in memory assessed by neuropsychological tests and assessment of change in clinical status. Secondary outcomes measures will include activities of daily living and behavioral/psychiatric aspects of the disease.